U.S. Frost Dates by Weather Station

NOAA station USC00241758 · Montana

Frost Dates for Circle, MT

NOAA weather station Circle (USC00241758), Montana · elevation 2480 ft (756 m) · hardiness zone 4a*

  • Lower-risk planting dateafter May 31only 1 spring in 10 still has a frost later
  • Typical last frostMay 1750% chance of a frost after it
  • Typical first fall frostSeptember 2250% chance of a frost before it
  • Frost-free season126 daystypical year, 32°F
  • Hardiness zone4acalculated*

The year at a glance

Frost season at Circle, MTSpring: a frost after May 4 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 22 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 126 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 17First frost ~Sep 22Frost season at Circle, MTSpring: a frost after May 4 in 9 years out of 10, after May 17 in half of the years, after May 31 in 1 year out of 10. Fall: the first frost comes before September 7 in 1 year out of 10, before September 22 in half of the years, before October 6 in 9 years out of 10. Typical frost-free season: 126 days.JFMAMJJASONDLast frost ~May 17First frost ~Sep 22
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 31September 7
50% (typical)May 17September 22
90% (likely)May 4October 6
Frost-free season (32°F, 50% chance)
126 days
Hard frost (28°F), last in spring, 50% chance after
May 6
Hard frost (28°F), first in fall, 50% chance before
October 1

The last 15 years (2006–2020 normals)

Typical last frost May 17 (same as the 30-year normal), typical first fall frost September 24 (2 days later). Frost-free season 128 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Circle: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 31, 20% May 26, 30% May 23, 40% May 20, 50% May 17, 60% May 15, 70% May 12, 80% May 8, 90% May 4. Observed last spring frost in recent years: 2021 May 11, 2022 May 22, 2023 May 19, 2024 May 4, 2025 May 2, 2026 May 19.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 31May 17May 42021: May 112022: May 222023: May 192024: May 42025: May 22026: May 19Fall at Circle: chance of a frost before this dateFall, chance of a frost before the date: 10% September 7, 20% September 13, 30% September 17, 40% September 20, 50% September 22, 60% September 25, 70% September 28, 80% October 1, 90% October 6. Observed first fall frost in recent years: 2021 September 17, 2022 September 21, 2023 October 6, 2024 September 27, 2025 October 6.Fall: first frostChance of a frost before this date10%50%90%Sep 1Oct 1Sep 7Sep 22Oct 62021: September 172022: September 212023: October 62024: September 272025: October 6
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021May 11September 17
2022May 22September 21
2023May 19October 6
2024May 4September 27
2025May 2October 6
2026May 19season not over yet

What these dates mean

Planting tender crops on May 4 in Circle is a gamble, because there is a 90 % chance of a light frost later than that date. The early spring date carries the highest risk, while the later May 31 date only leaves a 10 % chance of frost later, making it the cautious choice for setting out tender crops.

For gardeners, the 50 % dates are the points where a light frost is equally likely to have happened or not. The light-frost median in spring is May 17 and in fall is September 22. The hard-frost medians are May 6 in spring and October 1 in fall; these temperatures can cause more severe damage.

In the fall, September 7 is the cautious date, with only a 10 % chance that a light frost has arrived earlier, while October 6 represents a 90 % chance of a frost earlier, making it a gamble for keeping tender crops. A hard frost at 28 °F can injure plants that would survive a 32 °F light frost.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after May 31" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before September 7" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Circle, MTNumbered nearest stations: 1 — Brockway, MT — 14 mi; 2 — Vida, MT — 34 mi; 3 — Terry, MT — 45 mi; 4 — Glendive, MT — 46 mi; 5 — Cohagen, MT — 54 mi.25 mi · 40 km1. Brockway, MT — 14 mi12. Vida, MT — 34 mi23. Terry, MT — 45 mi34. Glendive, MT — 46 mi45. Cohagen, MT — 54 mi5
  1. Brockway, MT 14 mi (22 km)
  2. Vida, MT 34 mi (55 km)
  3. Terry, MT 45 mi (72 km)
  4. Glendive, MT 46 mi (75 km)
  5. Cohagen, MT 54 mi (87 km)

Sources

* Hardiness zone 4a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 47.4122, -105.5956.